Field of the Invention
The invention relates to a wave winding for a stator of an electric machine. The invention further relates to a stator of an electric machine. In addition, the invention relates to an electric machine.
Description of the Related Art
Known from printed publication DE 103 21 956 A1 are windings consisting of rectangular copper hairpins in several sets for an electric machine that exhibits a stator with several partly closed stator slots. In order to simplify production and to keep the slot-filling ratio of windings high, preformed rectangular wires with straight conductor segments are created which are arranged in the stator slots. For the purpose of producing the windings from rectangular copper hairpins, firstly the rectangular copper wire is cut into elements. In this case there are also two elements with shorter winding pitches (one slot shorter, compared with the others) per phase per winding set if the windings are connected in wave windings. Depending on the winding-pitch requirements, the rectangular wires are bent into U-shapes, and the copper hairpins are produced as a result. So-called “hairpin windings” are designated by the term “copper hairpins”. In this case, short winding pieces on one side of the stator are connected by welding, and in this way a continuous winding is generated. It is virtually a question of a constructed wave winding and not of a wave winding wound at least partly in uninterrupted manner.
Printed publication JP 8-182238 A presents a three-phase wave winding with three winding packets or winding strands for an electric machine. In a so-called “twisted portion”, winding heads of the winding packets or winding strands have each been twisted in themselves but guided with unchanged winding pitch.
A method for producing a lap winding for a dynamo-electric machine is known from U.S. Pat. No. 5,898,251 A. In a concentrically wound double-layer armature winding for the dynamo-electric machine the number of grooves per phase and pole is assumed to be q. The armature winding comprises a winding that corresponds to a pole and that includes a plurality of coils with winding pitches differing from one another. The coils include at least one coil having a number of turns that differs from those of the other coils. The number q of grooves per phase and pole amounts to at least q=3. The number of coils in a winding has been set to (q−n), where the number n takes the values n=1, 2, . . . q−2. The coils have been distributed in the grooves in such a manner that a concentrically wound double-layer winding with a sinusoidal distribution of magnetic force is formed. In a further arrangement, the number of pole windings per phase is half as large as the number of poles in a concentrically wound double-layer winding or in a lap winding. The number of coils in one of the pole windings is set to (2x(q−n)).
In order to satisfy certain requirements as regards torque and power with an electric machine, in the case of a predetermined overall length it is necessary to design the machine with a number of turns adapted to said length. If the machine has, in particular, been constructed with a wave winding, the number of turns thereof is determined from a number of conductor portions guided within a groove—for example, a stator groove, that is to say, a stator slot—of the machine and also from a number of conductors connected in parallel connection in the wave winding, in accordance with the formula:w=p*q*zn/a where    w is the effective number of turns of the machine,    zn is the number of conductors per groove,    a is the number of conductors connected in parallel connection in the wave winding,    p is the number of pole pairs and    q is the number of holes of the machine.
The term “number of holes” describes the number of grooves, for example stator grooves, of the machine per magnetic pole and phase. Consequently,s=2*p*q*r where    s is the total number of stator grooves of the stator and    r is the number of phases—that is to say, the phase number—of the machine.
In one example, a stator with a total of 96 grooves, three phases and 16 poles—that is to say, 8 pole pairs—possesses a number of holes q=2. The term “hole” is therefore synonymous with “groove” or “stator groove”.
Given a predetermined cross section of the groove, the number of conductor portions guided within a groove is inversely proportional to the conductor cross section of the conductor portions. However, a large conductor cross section results in increased frequency-dependent losses at high frequencies of the currents flowing through said conductor—that is to say, at high rotational speeds of the machine—by reason of the current displacement in the conductor in operation.
If, for example, 60 turns are to be produced, with p=10, q=2 and a=1, for example, it follows that zn=3—that is to say, three conductors are to be inserted into a groove. As a result, the cross section of an individual conductor and the frequency-dependent losses become large. Although in this example the number of conductors per groove could be doubled to zn=6 with q=1 and the cross section of the conductors could consequently be halved, the choice of the number of holes at q=1 has the disadvantage that harmonics arising in the machine are not suppressed, as a result of which increased losses and acoustic abnormalities—that is to say, increased operating noises—arise and, in addition, the torque ripple rises, which can likewise result in acoustic impairments.
By a setting of the structural design of the machine to a=1, the design scope for forming a desired number of turns is restricted. The variability of the machine is reduced; inter alia, the choice of possible configurations of the conductors is restricted. The requirements as regards torque and power of the machine cannot be satisfied, particularly within the range of high rotational speeds, by reason of the frequency-dependent losses associated with the large cross sections of the conductors. Therefore it is necessary to choose both the number of holes q and the number a of conductors connected in parallel connection in the wave winding to be greater than 1.
In the case of a machine with a wave winding in which several conductors have been connected to one another in parallel connection to form a phase, equalizing currents arise in the conductors of the parallel circuit in operation when voltages differing from one another are induced in the parallel conductors of a phase. This is the case, in particular, when the parallel conductors in the case of a number of holes q greater than 1 with respect to each pole have been arranged distributed to at least two adjacent grooves. The equalizing currents likewise result in losses.